The situation
After victory at Chancellorsville, Robert E. Lee took the Army of Northern Virginia north into Pennsylvania, hoping a win on Union soil would break the North's will. The two armies met almost by accident at Gettysburg, where many roads converged.
The decision
On day one the Confederates drove Union forces back through the town to the high ground of Cemetery Hill and Cemetery Ridge. On day two they attacked both Union flanks, including the fight for Little Round Top, without breaking the line.
On day three Lee ordered a frontal assault on the Union centre. James Longstreet argued strongly against it. About 12,500 men crossed nearly a mile of open ground under artillery and rifle fire. More than half were killed, wounded or captured.
What it cost
About 51,000 men were killed, wounded, captured or missing across three days. Lee retreated to Virginia and never again invaded the North. Four months later Lincoln delivered the Gettysburg Address on the battlefield.